Spassky has just dropped a bishop on h3 and the rook on f1 is attacked. Every white piece is still on the first two ranks, with a black queen, a rook and two knights aimed at the king. Before you move the rook, count Black's attackers: the knight on a5 is not one of them.
Diagram 18 · White to move
Petrosian vs Spassky, World Championship, Moscow 1966